In this work, we present the results of photoconductivity measurements performed in the temperature range of 12 K-300 K on a 150 nm-thick Bi 2 Te 3 film grown by molecular beam epitaxy on a (111) BaF 2 substrate. A transition from negative to positive photoconductivity is found to occur around 125 K. Resistivity and Hall data measured under light and dark conditions qualitatively elucidate the observed phenomena. The Arrhenius plot of recombination times obtained from photoconductivity decay curves measured at different temperatures gives the activation energy associated with the bulk trap level. Using this activation energy as the effective trap potential, we calculated the generation and recombination rates as a function of temperature. The analysis provides a quantitative explanation that predicts the transition effect observed in the experiment. No evidence of contribution from surface states is found from the magnetoresistance curves measured at low temperatures.
We have investigated the optical properties of GaMnAs/AlAs quantum wells (QWs) grown by molecular beam epitaxy under relatively high substrate temperatures (400 and 450 • C) and low Mn concentrations (0.1%). We have studied the time-and polarized-resolved photoluminescence emission as a function of the laser power and an applied magnetic field. Several anomalous results have been observed including long decay times, enhancement of the diamagnetic shift and reduction in the polarization degree with increasing Mn concentration.
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